Adam Rossington and Dan Lawrence flew out of the traps in pursuit of 146 in the final, but Henry dismissed both openers and produced the ball of the day with a bail-trimmer to see off Michael Pepper.
Daniel Sams threatened to pull off a dramatic finish with 45 off 26 balls but Henry, who was held back until the penultimate over, had the last laugh as Essex were all out for 131 to lose by 14 runs.
Somerset won their first crown in 2005 but they have been unrewarded on their previous seven visits to Finals Day, finishing runners-up in three successive years in 2009, 2010 and 2011 then again in 2021.
They reached the final by defending 142 against Surrey and needed another excellent collective effort from their bowlers after being bowled out for 145, built on Sean Dickson’s understated 53 off 35 balls.
Essex – who earlier edged out defending champions Hampshire on a day where both semi-finals were affected by passing showers, with the showpiece going off without a hitch – lost regular wickets as leg-spinner Ish Sodhi claimed three for 22 and Somerset captain Lewis Gregory took two for 25.
Essex’s Shane Snater was unfortunate to be on the losing side after exploiting the unpredictable bounce, perhaps caused by the moisture that built up through the day, to finish with 4-1-13-3.
Snater removed both openers before he had conceded anything off the bat, with Will Smeed inside-edging a leg-cutter on to his stumps for a run-a-ball nine in a wicket maiden while Tom Banton, dropped on nought, injudiciously reverse lapped a shot which looped gently to backward point for 20 off 16 deliveries.
Tom Kohler-Cadmore’s late cut brought the lone boundary Somerset took off Snater but he was out for a run-a-ball 19 after an attempted dab caught the under-edge on to the stumps.
Paul Walter was in the wickets when Tom Abell spliced to mid-on on the charge, attempting to up the ante after crawling to eight off 14 deliveries, while a flash point occurred when Gregory prodded his first ball back to Matt Critchley, who seemed to scoop up the catch diving forward.
Gregory was unconvinced and television replays only muddied the waters, with differing perspectives depending on the camera angle, persuading third umpire Nigel Llong to spare the Somerset captain.
To compound matters, Gregory edged Critchley for Somerset’s first four in 32 balls at the end of the 12th over. While Gregory did not especially excel after getting the benefit of the doubt, he proved a capable foil in a 46-run stand in 28 balls with Dickson, who upped the tempo after a watchful start.
Gregory was eventually out for 11 after miscuing Snater’s slower ball back to the bowler, starting a slide that saw Somerset lose their last six wickets for just 32 runs in 27 deliveries.
Dickson ploughed a lone furrow with a crucial 33-ball fifty – with most of his runs coming square of the wicket – but then gloved through to Rossington, who timed his jump to perfection, as Somerset only just batted out their allotment.
Rossington signalled his intent in the run chase by hammering the second ball back past Craig Overton for four as Essex’s openers took 27 off the first two overs. Rossington took three successive fours off Henry, who had his revenge as the opener departed for 19 off nine deliveries after bludgeoning to cover point.
Henry had two wickets in two deliveries after finding a hint of lateral movement to clip the off-bail of a stunned Pepper while later in the over, Lawrence, who was also in a hurry after slamming Overton over long-on, was beaten for pace and rapped on the front pad for 16 off eight balls.
Overton got in on the act as Robin Das got no elevation on a straight drive while Henry was unfortunate not to make further inroads in his third over after beating the outside edge of Walter and Critchley.
Walter batted sensibly for 26 off 24 balls to keep Essex up with the rate but was castled after missing a heave off Sodhi.
While Essex continued to lose wickets, Sams made sure they did not collapse without trace by clattering Kasey Aldridge and Sodhi over the boundary rope to leave 21 required off the final 12 balls.
It was all on his shoulders as he greeted Henry back into the attack with a third towering six but when the New Zealand seamer went wider, Sams got a thick edge and Kohler-Cadmore produced a terrific diving catch as Essex were all out in 18.3 overs.